Chloromethylation Modified Pyranonitrile‐Based Conjugated Microporous Polymers for Selective One‐Step Two‐Electron O ₂ Reduction to H ₂ O ₂

Hydrogen peroxide (H 2 O 2 ) production utilizing conjugated microporous polymers (CMPs)‐based photocatalysts represents a crucial green technology for achieving solar‐to‐chemical energy conversion. Proper material design is paramount to improve the dispersity and charge transfer of CMPs for enhanced H 2 O 2 production performance. Herein, a post‐modification strategy employing chloromethylation reaction was proposed to enhance H 2 O 2 production. The simple one‐step chloromethylation reaction simultaneously achieved two objectives: One is enhanced hydrophilicity through the hydrolysis of cyanogen groups in the pyranonitrile unit to carboxyl groups, the other is the improved O 2 adsorption and charge transfer by incorporating chloromethyl groups. The two objectives synergistically enhanced the H 2 O 2 production rate of the chloromethylated CMP named DCM‐TPA‐Cl, reaching 5.01 mmol g −1 h −1 in air, 6.7‐fold of the unmodified photocatalyst. Moreover, the rate achieved at an O 2 atmosphere increased by only 1.8%, highlighting its superior O 2 utilization efficiency in air. An exceptional 38.02 mmol g −1 h −1 rate was further achieved in water/benzyl alcohol mixtures, exceeding most reported polymer photocatalysts. Experimental and theoretical results corroborated the predominant role of the one‐step two‐electron O 2 reduction pathway in the H 2 O 2 generation. This work demonstrates the potential of a post‐modification method to significantly enhance H 2 O 2 production performance directly from water and air.